(1) Field of the Invention
The present invention relates to a liquid crystal composition. More particularly, the present invention relates to a liquid crystal composition in which an improved orientation, high contrast, high memory stability, and high-speed response are obtained by using a conventional liquid crystal and a liquid crystal having a specific naphthalene structure as main components, and mixing same at a predetermined ratio.
Furthermore, the present invention relates to a ferroelectric liquid crystal display device comprising the above liquid crystal composition.
(2) Description of the Related Art
Since a liquid crystal display element is a plate type element with a small power consumption, the liquid crystal display element is widely used as a display element of a word processor, a lap top computer or the like. Especially, a super twisted nematic (STN) liquid crystal display element can be applied to a display having a relatively large display capacity such as 640.times.400 dots, and is widely used for personal computers and the like.
Nevertheless, as the display capacity of the liquid crystal display increases and applications to word processors or personal computers having an enhanced performance are attempted, the application of a conventional STN becomes difficult because it has an unsatisfactory display background color and viewing field angle.
As a display system proposed for overcoming these defects of the conventional liquid crystal display, a ferroelectric liquid crystal display (FLCD) was proposed [N.A. Clark and S.T. Largerwall, J. Applied Physics Letters, 36, 899 (1980)].
Also U.S. Pat. No. 4,367,927 (Noel A. Clark) discloses "Chiral Smectic Cor H Liquid Crystal Electro-Optical Device", wherein a liquid crystal electro-optical device comprising:
a quantity of chiral, at least one of smectic C and smectic H, liquid crystal having a plurality of adjacently disposed layers each comprised of a plurality of molecules, each molecule having a long axis;
first and second means, perpendicular to and contiguous with said layers, for aligning the long axes of said molecules adjacent to said first and second means parallel to said first and second means, said molecules of said layers in a bulk of said liquid crystal forming helixes having axes perpendicular to said layers, the distance between said first and second means being less than the distance at which said helixes form in the absence of an electric field, said first and second means causing said long axes to assume one of a first and second stable orientation;
means for applying an electric field to at least a portion of said layers, said electric field being parallel to said layers and perpendicular to said first and second means, and for reversing the direction of said electric field, the strength of said electric field being sufficient to shift the long axis of molecules in said portion of said layers from said first orientation to said second orientation upon application of said electric field in a direction reversed from the direction of said electric field previously applied; and
means for processing light passing through said liquid crystal so that light passing through portions of said liquid crystal having said first orientation can be distinguished from light passing through portions of said liquid crystal having said second orientation, indicated.
FLCD shows a spontaneous polarization because the polarizations of liquid crystal molecules are oriented in one direction, and since this spontaneous polarization is reversed when the polarity of the applied electric field is reversed, FLCD shows ferroelectric characteristics. Accordingly, as the drive method, a method is adopted in which the spontaneous polarizations of liquid crystal molecules are oriented in one direction by applying a pulse wave form electric field having a negative polarity, the display contents are held by the memory effect of the liquid crystal, i.e., the property whereby after the orientation of the polarizations, the ferroelectric characteristic, that is, the direction of the spontaneous polarization, is not changed, and the direction of the spontaneous polarization is first changed by an application of an electric field having a reverse polarity. Therefore, in FLCD, the memory effect must be kept stable, but in the conventional ferroelectric liquid crystals, a satisfactory memory effect necessary for stable driving cannot be obtained, or if obtained, the response time is long and the characteristics of FLCD are not fully utilized.
A surface stabilized ferroelectric liquid crystal display (SSFLCD) is recognized as having a great possibilities with regard to a high information content, wide viewing angle, high contrast ratio, and fast switching. To realize a high performance SSFLCD, much research has been carried out into the development of liquid crystal materials, driving methods, and liquid crystal molecular orientations.
Nevertheless, the SSFLCD is still under development, mainly because the layer structure of the SSFLCD is a "chevron" not a "bookshelf" structure, and because of this structure, the SSFLCD shows "zig zag defects", (Y.Ouchi, J. Lee, H. Takezoe, A. Fukuda, K. Kondo, T. Kitamura and A. Mokoh, J.J.A.P. 27 L1993 (1988)), resulting in a poor contrast ratio and unstable bistability.
A "bookshelf" structure obtained by an oblique deposition technique has been reported (M. Johno, A. D. L. Chandani, Y. Ouchi, H. Takezoe, A. Fukuda, M. Ichihashi and K. Furukawa, J.J.A.P. 28 L119 (1989)), but a bookshelf structure with a rubbed polymer film cell and suitable for a mass production of the SSFLCD has not been reported.
Accordingly, the development of a liquid crystal material maintaining a high-speed response characteristic, a characteristic of FLCD, and having a satisfactorily stable memory effect is desired in the art.
A mixture composed mainly of an ester compound and a phenylpyrimidine compound is mainly used for the conventional FLCD, but a satisfactorily stable memory effect cannot be obtained by any of the known compositions of this type. The main reason for this is that, since a compatible Coulomb mutual action (interaction) is not effected in the interface between each liquid crystal molecule and the substrate, microdomains are formed, or if a certain orientation is obtained, many zigzag defects are generated as described hereinafter.
Under this background, the inventors proposed an FLCD in which a high-speed response characteristic and a stable memory effect can be manifested by using a naphthalene type liquid crystal having a naphthalene ring in the core unit portion of the liquid crystal molecule (Unexamined Japanese Patent No. TOKKAIHEI 1-101389). The naphthalene type liquid crystal is a material which is different from conventional FLCD, in that the generation of zigzag defects is not observed, and in which a liquid crystal orientation having an excellent uniformity can be manifested, and further, the naphthalene type liquid crystal has a much better memory characteristic than those of the conventional liquid crystal materials.
In general, however, the naphthalene type liquid crystal has a high viscosity, and an increase of the viscosity is especially conspicuous at temperatures lower than 15.degree. C., and therefore, the naphthalene type liquid crystal is defective in that the response time is abruptly lengthened in a low-temperature range (lower than 10.degree. C.).
Accordingly, the inventors searched for a liquid crystal material retaining the characteristic of conventional liquid crystals, i.e., a relatively low viscosity in a low-temperature region, and having the good orientation and memory characteristics inherently possessed by the naphthalene type liquid crystal. First, a mixture of a conventional phenylpyrimidine type mixed liquid crystal with the naphthalene type liquid crystal was examined, and as a result, it was confirmed that the compatibility between the phenylpyrimidine liquid crystal and the naphthalene type liquid crystal is generally poor and the ferroelectric phase-showing temperature range is narrowed by the mixing. Accordingly, the inventors tried to obtain a stable memory characteristic in a broad ferroelectric phase-showing temperature range by mixing various naphthalene type liquid crystals into conventional liquid crystals as the base, but it was found that naphthalene type liquid crystals differ in their compatibility with the conventional liquid crystals according to the difference of the molecular structure, and that the intended effects of the present invention can be attained by a novel liquid crystal composition formed by combining the liquid crystals described below. The present invention is based on this finding.
The research objectives of the present inventors were as follows.
First, to clarify the relationships between the molecular structure and layer structure of the liquid crystal, and then to obtain a bookshelf structure; second to investigate dependence of the layer structure on the electro-optical properties thereof; and third, to obtain a high contrast ratio and stable bistability with the bookshelf layer structure of the SSFLCD with a rubbed polymer film.